1. Field of the Invention
This invention relates to electrode apparatuses used in detecting bioelectric signals on a living body, especially for use in detecting electromyographic (EMG) signals from the skin surface of a human body.
2. Description of Related Art
Electrodes which are conventionally used for the acquisition of EMG signals are either invasive (needle or wire) electrodes which penetrate through the skin into a muscle, or non-invasive surface electrodes which are placed on the skin surface above a muscle. Surface electrodes are preferred for their non-invasive characteristics, but their usefulness is limited. One of the reasons for this is that they pick up concurrent activities of many muscle fibres, yielding complex action potentials which represent summated or averaged activities. Reportedly EMG signals can be obtained selectively from the skin surface, i.e. action potentials of muscle fibres of individual or a small number of motor units can be obtained with appropriate combinations of several surface electrodes. The structural and dynamic properties of a muscle are detectable with use of groups of similar electrodes appropriately combined. Many basic electrodes are usually combined at fixed distances from each other in one or two dimensions and are conventionally called an electrode array.
The basic electrodes which form the array must have small contact areas with the skin. Pin electrodes with flat tips have been used in the past in the construction of arrays. However such electrodes have not been very successful because the skin-electrode impedance is too high. Pin electrodes with sharp tips have also been used to investigate the skin electrode impedance with different penetrating depths of the needle point into the skin. The results indicate that the deeper the penetrating depth, the lower the skin-electrode impedance.
U.S. Pat. No. 4,685,466 discloses a quasi-invasive needle-like electrode which penetrates into the upper, most largely cast-off cell layers of the horny skin to reduce the skin electrode impedance in a very small contact area. Since the contact area between the electrode and the skin is so small, a minimum depth of penetration must be achieved by the electrode in order to reduce the skin-electrode impedance to an acceptable level. On the other hand, the depth must be limited to the thickness of the layer of the stratum corneum of skin (from less than 50 to over 350 micrometers on human skin) in order to keep the electrode non-invasive. However it is difficult or at least inconvenient to determine and control the depth of the penetration of such an electrode in practice, especially when the electrode is part of an array and when motion occurs on the skin or in the muscle. In addition, regular electrolytic treatment is also necessary for electrodes of such small contact area to reduce the impedance of the electrodes themselves.
An example of an electrode array is found in U.S. Pat. No. 4,969,468 to Byers et al. The electrode array includes a base and a plurality of electrically conductive protuberances extending substantially perpendicular to and from the support surface of the base in a two dimensional array. Each of the protuberances has a tip for electrically contacting the tissue.
The prior art discloses various ways to improve types of biological electrodes other than those for electrode arrays suitable for EMG electrodes. Such other types typically have much larger skin contact areas (usually greater than 5 mm.sup.2). An example is found in U.S. Pat. No. 3,882,846 to Fletcher. The electrode system includes an insulated electrode and an impedance transformer. Another example is found in U.S. Pat. No. 4,706,679 to Schmidt. This electrode comprises bundles of silver wire held by a conductive tube with a flat tip. These however are large area electrodes which are intended to be used individually and not in an array.